KANSAS CITY, Mo., April 22 (Reuters) - The U.S. Supreme Court on Tuesday refused to stay the execution of a death row inmate in Missouri whose attorneys had argued that Missouri's secrecy around its lethal injection drugs could result in undue suffering.

The high court's decision not to take the case comes a day after the Oklahoma Supreme Court stopped two executions there over similar issues.

William Rousan, 57, is scheduled for execution at 12:01 a.m. Central Time (0501 GMT) on Wednesday. Rousan was convicted of murdering 62-year-old Grace Lewis and her 67-year-old husband, Charles Lewis, in 1993 in a plot to steal the farm couple's cattle.

Eric Butts, an attorney representing Rousan, said there were no more avenues for appeal. "This is it," said Butts. "They (the Supreme Court) just are really not interested in the situation."

In their petition to the U.S. Supreme Court, Rousan's attorneys said the state was planning to use "compounded pentobarbital prepared by an unknown person in an unknown manner, without any assurance by an accredited laboratory that the substance is what the state purports it to be."

Last year, Missouri started classifying compounding pharmacies as part of its execution team and said the identities of the pharmacies were thus shielded from public disclosure.

But Rousan's attorneys argued that Rousan has a right to know what he will be injected with.

On Monday, the Oklahoma Supreme Court halted the executions of Clayton Lockett, scheduled for Tuesday, and Charles Warner, scheduled for April 29. The court said the inmates had the right to have an opportunity to challenge the secrecy over the drugs Oklahoma intends to use to put them to death.

On Tuesday, Oklahoma Attorney General Scott Pruitt filed a motion in the Oklahoma Supreme Court seeking a rehearing on the stay of execution order.

Many states have turned to the lightly regulated compounding pharmacies for supplies because makers of drugs traditionally used in lethal injections have largely stopped making them available for executions.

But lawyers for death row inmates in several states have argued that drugs obtained for lethal injections from compounding pharmacies could lead to undue suffering, which would amount to cruel and unusual punishment in violation of the U.S. Constitution. They also say they should have information about the legitimacy of the supplier and details about the purity and potency of the drugs.

Prison officials have rejected those arguments and have been refusing to reveal where they are getting the drugs.

Louisiana and Ohio, however, have seen executions delayed this year because of concerns about suffering that might be caused by untraditional drug supplies. The family of one inmate executed in Ohio in January has filed suit against the state because, according to some witnesses, he took an unusually long time to die and appeared to be in pain. (Reporting by Carey Gillam; Additional reporting by Heide Brandes in Oklahoma City; Editing by Mohammad Zargham and Cynthia Osterman)